The ‘Temperature Integral’ — Its use and abuse

9 2000 Elsevier Science B.V. All rights reserved.

295

Geological Exploration in Murzuq Basin M.A. Sola and D. Worsley, editors.

CHAPTER 14

The structure, stratigraphy and petroleum geology of the Murzuq Basin, southwest Libya LINDSAY DAVIDSON, 1 SIMON BESWETHERICK, 2 JONATHAN CRAIG, 3 MARTIN EALES,1 ANDY FISHER, 1 ALI HIMMALI,5 JHOON JHO, 4 B A S H I R M E J R A B 5 and JERRY S M A R T 2

ABSTRACT The Murzuq Basin covers an area of over 350 000 km 2, and is one of several intracratonic basins located on the North African Platform. The present-day borders of the basin are defined by tectonic uplifts, each of multi-phase generation, and the present basin geometry bears little relation to the much broader North African sedimentary basin which existed during the early Palaeozoic. Several generations of fault movement are recognised in the basin, but the resultant degree of deformation is relatively minor. The basin contains a sedimentary fill that reaches a maximum thickness of about 4000 m in the basin centre and comprises a predominantly marine Palaeozoic section and a continental Mesozoic section. The principal hydrocarbon play in the basin consists of a periglacial sandstone reservoir of Ordovician age sourced and sealed by overlying Silurian shales. This play has proved very successful and accounts for approximately 1500 million barrels of recoverable oil discovered to date. Thermal modelling presented in this chapter suggests that the main phase of oil generation may have taken place during the Cretaceous, but further work is required to better define the timing of oil charge. Subsequent regional uplift and erosion has resulted in cooling of the source rocks, which are no longer generating oil over large parts of the basin At the present day the Silurian source rock remains within the oil generating window only in a limited area of the basin centre. The key to better understanding of this play is the relative timing of oil generation compared to Cretaceous and Tertiary inversion tectonics which influenced burial depth of the source rock, reactivated faults on the trapping structures and reorganised migration pathways. Many of the discovered fields and exploration prospects identified in the Murzuq Basin involve high angle reverse faults, and are typically found in the hanging wall or in tip-line folds above the faults. Fault orientations in the basin show considerable variation, but a dominant clustering around a north-south trend suggests the influence of a late

1 LASMO Grand Maghreb Limited, 101 Bishopsgate, London, EC2M 3XH, UK. Email [email protected] 2 LASMO Oil Pakistan Limited, Sasi Arcade, No. BC5, Block No. 7, Clifton, Karachi, Pakistan 3 LASMO plc, 101 Bishopsgate, London EC2 M3XH, UK 4 Korea National Oil Corporation, 1588-14 Kwanyang-dong, Kyungki-do, Korea 5 LASMO Grand Maghreb Limited, Tower 4, Dat El-Imad, Tripoli, Libya.

L. Davidson et al.

296

Precambrian Pan-African grain in the underlying basement. Initial Palaeozoic fault movements are recognised in the Cambro-Ordovician, with significant growth occurring across steeply dipping faults. Subsequent reactivation during late Silurian to early Devonian compression resulted in reverse displacement on many of the larger faults, creating the presently observed trapping structures. Further reverse fault movements and transpression also occurred during the mid-Carboniferous, mid-Cretaceous (Austrian) and early Tertiary (Alpine) compressive tectonic phases, all of which were associated with regional uplift and erosion.

EXPLORATION HISTORY Exploration activity began in the Murzuq Basin in the 1950s and has carried on sporadically since. The B 1-1 well drilled in 1957 on the Atshan Arch to the northwest of the Murzuq Basin was the first exploration well to discover hydrocarbons in Libya, but appraisals of this gascondensate discovery proved to be disappointing. Subsequent success in the Sirt Basin in the 1960s diverted attention away from the more remote Murzuq areas and it was not until the 1980s that exploration activity picked up once again with Braspetro, Rompetrol and BOCO taking exploration licenses in the basin. Braspetro drilled eight wells in NC58 (Meister et al., 1991; Pierobon, 1991) without encountering any commercial accumulations, although their first well, A1-NC58, was a small discovery. BOCO made several oil discoveries in NC101, but these have not been developed to date and are relatively small. Rompetrol was extremely successful in block NC115, with their twelve well exploration programme resulting in the discovery of three large fields, 'A', 'B' and 'H'. Operatorship of this block was subsequently transferred to Repsol who initiated a development programme for these fields, with production beginning in 1997. The oil is transported through a newly constructed pipeline to the Hamada field, from which an existing pipeline takes it to the oil terminal at Zawiyah on the coast west of Tripoli. In 1990, a consortium of companies led by Pedco (now KNOC) was awarded the NC174 license, located between the NC101 and N C l l 5 blocks. LASMO Grand Maghreb Ltd. subsequently joined Pedco to act as operator for exploration of the acreage. Four wells drilled during 1993 and 1994 resulted in two small oil discoveries in Mamuniyat sandstone reservoirs. One of the discoveries also encountered a small oil column in Devonian sandstone. Agip North Africa BV joined the NC174 group in 1996 and a new exploration programme began in the block during 1997. The second well in this campaign, F1-NC174, drilled on the 'Elephant' prospect, encountered a significant oil accumulation, which has subsequently been successfully appraised. The Elephant discovery is currently estimated to contain over 500 million barrels of recoverable oil and at the time of drilling was the largest oil discovery in Libya for fourteen years.

REGIONAL TECTONIC SETTING The Murzuq Basin is one of several intracratonic basins located on the North African Platform. The basin covers an area of over 350 000 km 2, and has a roughly triangular shape, narrowing towards the south from Libya into Niger (Fig. 1). It is not a sedimentary basin in the normally accepted sense, and could more accurately be described as an erosional remnant of a much larger Palaeozoic and Mesozoic sedimentary basin which originally extended over much of North Africa, as described by many authors (e.g. Boote et al., 1998). The present-day borders of the

Chapter 14

297

Murzuq Basin are defined by erosion resulting from multiphased tectonic uplifts, the flanks comprising the Tihemboka High to the west, the Tibesti High to the east and the Gargaf/Atshan Uplift to the north. These uplifts were generated by various tectonic events ranging from mid Palaeozoic through to Tertiary times, but the main periods of uplift took place during midCretaceous (Austrian) and early Tertiary (Alpine) movements. There is little evidence that these present basin-bounding uplifts were active during the early Palaeozoic, when upper Ordovician sandstone reservoirs and lower Silurian source rocks of the primary play system were deposited. The main influence on sedimentation at that time was probably exerted by the NW-SE trending Tripoli-Tibesti Uplift (Klitzsch, 1971), which extended across the northeastern part of the present-day basin. The influence of this palaeohigh is demonstrated by a progressive thinning of the Silurian Tanezzuft Formation shale towards the

Figure 1. Simplified map of the surface geology of the Murzuq Basin, showing current exploration and production license areas. Source: Geological map of Libya, Industrial Research Centre, 1985. International borders from NOC Concession Map, 1994.

298

L. Davidson et al.

northeast of the basin, mainly as a result of erosion, with the unit being demonstrably absent along the exposed southern margin of the Gargaf Uplift. The erosional nature of the basin margins and the partial erosion of the Tanezzuft shale are illustrated in the N-S cross-section shown in Fig. 2. The current state of preservation of the basin probably relates to the underlying lithosphere, which is inferred to be relatively competent and less susceptible to compressive stress compared to that beneath the surrounding highs, which have been preferentially uplifted and eroded. The present-day basin centre contains a maximum sedimentary fill of about 4000 m. In addition to this a cumulative total of 1000 to 2000 m of section may have been stripped off during several phases of uplift and erosion throughout the history of the basin, but the maximum sedimentary thickness probably never exceeded 5000 m at any single point in time. Overall, this is a relatively modest sedimentary accumulation for a basin of some 500 million years in age. The principal reason for this is that accommodation space in the Murzuq Basin has been created primarily by sag processes, and the basin has never been subjected to any significant phase of extensional rifting, in marked contrast to the Sirte Basin which lies only some 400 km to the northeast. Several generations of structuring, mainly compressional and transpressional in nature, are recognised within the Murzuq Basin, but the cumulative structural deformation is relatively minor. No quantitative estimates of regional strain have been published for this area, but it is probable that overall crustal shortening across the basin is less than 1% in any orientation. It is this crustal and lithospheric stability which has preserved the lower Palaeozoic source and reservoir rocks from excessive burial, while also resisting trap-destroying uplift and erosion. This has allowed the development and retention of a productive hydrocarbon system in rocks considerably older than the average for successful petroleum provinces. Seismic evidence shows only one possible phase of extensional fault movement within the basin, during Ordovician times, but these faults are widely spaced and of relatively small displacement, usually < 100 m. Other phases of extensional movement may have taken place, but if so, these were never of major significance and their effects are now obscured by subsequent compressional and/or transpressional faulting and fault reactivation. The depositional history of the basin has been punctuated by various phases of uplift and erosion, principally during the Cambro-Ordovician, the late Silurian/early Devonian, middle and late Carboniferous, mid-Cretaceous and early Tertiary, as detailed below. Evidence from apatite fission track work, fluid inclusion data and shale velocity studies (Glover, 1999) indicate that the early Palaeozoic rocks encountered in wells in the basin are not presently at their maximum burial depths, but have been subjected to moderate but significant uplift and cooling since these maximum depths were reached. A cross-section (Fig. 2) shows present-day surface elevations to be 500 to 900 rn above sea level, while there is clear evidence of ongoing active erosion, most noticeably at the scarp of the Jurassic to lower Cretaceous Mesak Formation. The current aeolian depocentres (sand seas) within the basin can be regarded as temporary ponded sub-basins which are unlikely to be preserved in the future geological record. These will themselves be removed by erosion once the threshold barriers of the Gargaf Uplift and the Mesak scarp are progressively worn down and sediment transport routes become re-established towards the north.

BASIN STRATIGRAPHY A stratigraphic column for the Murzuq Basin is shown in Fig. 3. Previous syntheses of available outcrop data from the margins of the basin have been published by Mamgain (1980), Bellini and Massa (1980) and Abugares and Ramaekers (1993), among others. The stratigraphic schemes

:r. t~ 4~

Figure 2. NNE-SSW trending geological cross-section through the Murzuq Basin, constructed from outcrop, well and seismic information. See Fig. 1 for location.

t,,.) ~D

300

L. Davidson et al.

established by these authors have been slightly modified in Fig. 3 to incorporate additional information from the subsurface of the present basin centre. The sedimentary deposits in the Murzuq Basin range from Cambrian to Cretaceous in age, and can be divided into four major sedimentary units.

Figure 3. Summary of the stratigraphy, hydrocarbon play systems and chronology of tectonic events in the central part of the Murzuq Basin.

Chapter 14

301

Cambro-Ordovician A generalised stratigraphy of the Cambro-Ordovician is shown in Fig. 4. The first sediments to be deposited throughout the basin belong to the Cambrian Hasawnah Formation. A basal conglomerate has been recorded, but most of the formation comprises medium to very coarsegrained, quartzitic sandstone. The environment of deposition passed from fluvial at the base of the formation to shallow marine at the top. Sediment supply was from the south, with the sea transgressing from the north. An unconformity separates the Hasawnah Formation from the overlying Ordovician Hawaz Formation. The type section of the Hawaz Formation on the Gargaf Uplift consists o f fine to medium-grained sandstone, with subordinate siltstone and shale. This section has been described by Vos (1981), who suggested that the sediments were deposited in a fan delta complex which prograded across the Gargaf Uplift in a northerly direction. The Ash Shabiyat Formation is probably the lateral equivalent of the Hawaz Formation on the Tihemboka High. In the northwestern part of the basin, the Hawaz Formation is overlain by shales of the upper Ordovician Melaz Shuqran Formation. This fairly distinctive unit, with uniform wireline log

Figure 4. A summary of the Cambro-Ordovician stratigraphy, lithology and depositional settings in the Murzuq Basin. The section represents an idealised column with approximate maximum thicknesses of the constituent formations. In NC174 the Cambro-Ordovician is generally thinner than illustrated here; parts of the succession are often missing, either because of erosion or non-deposition.

302

L. Davidson et al.

responses, has been dated to the Ashgill (Abugares and Ramaekers, 1993). The shales were probably deposited in a relatively shallow marine environment, and a predominantly green colour might indicate reducing conditions, suggesting restricted marine circulation. On the Tihemboka High, the presence in the upper few metres of the formation of siltstone and very fine grained sandstone beds which show soft sediment deformation, suggests that the contact with the overlying Mamuniyat Formation is transitional in nature, with no major break in sedimentation (Beswetherick et al., 1996). The Melaz Shuqran Formation is frequently absent in wells drilled in the centre of the basin, but it is not clear whether this is a result of nondeposition or of subsequent erosion. The uppermost Ordovician sediments of the Mamuniyat Formation form the primary hydrocarbon reservoir in the basin, and the reservoir properties and characteristics of this formation are described below in more detail. The Mamuniyat Formation consists mainly of sandstone with subordinate siltstone and shale beds, also dated to the Ashgill (S.P.T., 1994). The sandstone is typically quartzitic, fine to medium-grained, and fairly well sorted. Several different facies can be recognised in the Mamuniyat Formation, although most can be assigned to a high energy, deltaic to marine environment of deposition. The Mamuniyat Formation and underlying Melaz Shuqran Formation were deposited at a time of glaciation over North Africa, which lay along the margin of Gondwanaland. It is probable that the ice sheet periodically extended as far north as the Murzuq Basin. In the Ghat area on the western flank of the basin the authors have observed some direct evidence for glaciation in the presence of small dropstones in the Melaz Shuqran shale and the occurrence of interpreted ice striations on bedding planes within the Mamuniyat Formation. The action of glaciation in the hinterland to the south was important in releasing vast quantities of sediment, which were initially transported into the area of the Murzuq Basin by high energy braided or sub-glacial fluvial systems, and then reworked in a marine environment, often as gravity flow deposits. Seismic evidence from the subsurface in NC174 suggests that the upper part of the Mamuniyat Formation may contain a series of deeply incised erosional channels filled with fluvioglacial sediments (Smart, 2000). The late Ordovician glacial period would also have resulted in repeated short lived sea level changes, connected with the growth and retreat of the ice cap (Hadley, 1992), providing the alternating marine and fluvial influences observed in deposits of the Mamuniyat Formation. One general comment regarding the stratigraphy of the Cambro-Ordovician in the Murzuq Basin is that the sand-rich facies that dominate this section tend to provide very poor biostratigraphic data and age dating is therefore very poorly constrained. Formational identifications in the subsurface are usually based on lithostratigraphic or log criteria, often quite unreliable for regional correlation.

Silurian to Devonian

The Silurian began with a major marine transgression which spread from the north, and reached across much of the North African margin. An unconformity separates the Mamuniyat Formation from the overlying shale of the Silurian Tanezzuft Formation. An organic and uranium rich 'hot shale' with patchy areal distribution within the basal part of the Tanezzuft Formation forms the main hydrocarbon source rock within the basin. The hot shale seldom exceeds 50 m in thickness, although the overall thickness of the whole Tanezzuft Formation may be more than 800 m. The lower parts of the Tanezzuft Formation have been dated to the early Llandovery. The nature and distribution of this hot shale is discussed in more detail below. The Tanezzuft shales thin and become more arenaceous towards the northeast of the basin in the direction of the Tripoli-Tibesti palaeohigh, which clearly exerted an influence on early

Chapter 14

303

Silurian deposition. The sandy, shallow marine mid- to upper Silurian Akakus Formation, which outcrops along the western basin margin, is apparently absent in wells at the present-day basin centre, probably as a result of early Devonian erosion. The Tanezzuft Formation is unconformably overlain by the mid- to upper Devonian Awaynat Wanin Formation. Sediments of early Devonian age appear to be absent in the basin centre. The Awaynat Wanin Formation comprises shale and subordinate sandstone, often rich in iron, deposited in a littoral to shallow marine environment. The sandstones of this formation constitute the reservoirs for a secondary hydrocarbon play in the basin.

Lower to Mid-Carboniferous The early to mid-Carboniferous was also marked by a significant marine transgression. The Awaynat Wanin Formation is overlain by the Marar Formation, which is in turn overlain by the Assedjefar Formation. Both of these formations are early Carboniferous in age and consist of shale with subordinate sandstone. Several minor coarsening-up cycles can be traced across the basin, suggesting deposition in a relatively low energy shallow marine environment with periodic shoaling. The overlying mid-Carboniferous Dembaba Formation comprises shallow marine limestone, sandstone and grey shale in the northern part of the basin, and lagoonal limestone and red shale in the southern part. This formation represents a transition from the marine conditions typical of much of the Palaeozoic, to the continental conditions which have prevailed since. Red lacustrine mudstone of the upper Carboniferous Tiguentourine Formation may be found in parts of the basin but this unit is often absent, either because of non-deposition or as a result of uplift and erosion during the late Carboniferous - which produced the so-called Hercynian unconformity. Permian sediments are not present in the basin, probably due to non-deposition following this late Carboniferous regional uplift.

Triassic to Lower Cretaceous Triassic to lower Cretaceous sediments were deposited in continental conditions. This section can be divided into the fluvial sandstone and red mudstone of the Triassic to Jurassic Zarzaitine/ Taouratine formations and the fluvial sandstone, conglomerate and mudstone of the Jurassic/lower Cretaceous Mesak Formation. These formations appear to be conformable but their contact may represent a significant period of non-deposition. Approximately 1700 m of Mesozoic section are preserved in the basin centre, but it remains uncertain how much Cretaceous and Tertiary succession has been removed by erosion following the mid-Cretaceous (Austrian) and early Tertiary (Alpine) uplifts.

TIMING OF BASIN S T R U C T U R I N G - IMPLICATIONS FOR PETROLEUM EXPLORATION A chronostratigraphic chart (Fig. 3) shows the principal stratigraphic units and hydrocarbon plays in the Murzuq Basin. Also indicated is the timing of tectonic events which influenced the basin's development and which were responsible for the major erosive unconformities which we now observe. Numerous minor unconformities and disconformities are also present within the stratigraphic section.

304

L. Davidson et al.

Cambro-Ordovician Faulting The earliest recognised structures to affect the sedimentary section of the basin are faults of Cambro-Ordovician age. Many of these were subsequently reactivated by later movements and this can lead to some difficulties in interpretation of Cambro-Ordovician kinematics. A few small displacement features that have not undergone post-Ordovician reactivation can be interpreted as extensional normal faults with thickening of the hanging wall Cambro-Ordovician section, as illustrated in Figs 5 and 6. If this interpretation is correct, these are the only purely extensional faults yet recognised in the basin, but with typical minor displacements of less than 100 m. More commonly, however, thickening of the Cambro-Ordovician section is observed in the footwall of steeply dipping reverse faults, implying synsedimentary movement of compressional or transpressional origin, as seen in the main block-bounding faults in Fig. 6. Displacements are significant, often with over 100 m thickening across the larger faults. Well D1-NC174 (Fig. 6) suggests that these fault movements predated deposition of the upper Ordovician Mamuniyat Formation, the main reservoir rock in the basin, which is present on the crest of the horst block despite severe thinning of the overall Cambro-Ordovician section.

Silurian to Devonian (Caledonian) Tectonics A major unconformity in the basin reflects late Silurian to early Devonian tectonic movements, which were mainly of a compressional nature. Bellini and Massa (1980) state that these

Figure 5. Seismic section trending NW-SE and passing through the Elephant Field discovery well, F1-NC174.

Figure 6. Seismic section trending NW-SE and passing through the D 1-NC174 dry hole.

306

L. Davidson et al.

movements persisted from mid- to late Silurian times, and in some outcrops an angular unconformity of up to 5 ~ can be observed between Silurian and overlying Devonian strata. Chronostratigraphic sections published by Boote et al. (1998) and Abugares and Ramaekers (1993) show much of the upper Silurian and lower Devonian section to be missing through erosion at this unconformity. The thick upper Silurian sandstones of the Akakus Formation, which outcrop on the western flank of the basin, are apparently absent in wells in the basin centre, probably due to erosion. In the adjacent Ghadames Basin, Echikh (1998) shows evidence of basinwide partial erosion of the Akakus Formation at the late Silurian to early Devonian unconformity. Many of the structures which form present-day hydrocarbon traps in the basin were initiated by late Silurian to early Devonian compression. The seismic line in Fig. 5 illustrates the trapping structure of the F1-NC174 discovery that was created by a high-angle reverse fault. The Silurian/ Devonian section shows significant thickening across this trap-forming fault, indicating synsedimentary fault movement at this time. Later reactivation of this fault occurred during midCarboniferous and Cretaceous/Tertiary movements. More precise dating of the movements on this fault require drilling and detailed biostratigraphic analysis of the thickened footwall sedimentary section, but such data are not available at the present time due to a lack of well penetrations. The available seismic evidence does, however, indicate that every period of movement on this fault appears to have been compressional or transpressional in origin. The late Silurian to early Devonian faults in the Murzuq Basin vary in trend from NW-SE through N-S to NE-SW, probably following pre-existing Pan-African trends. Underlying the northeastern flank of the basin is the Silurian age NW-SE trending Tripoli-Tibesti Uplift (Klitzsch, 1971), also termed the Bin Ghanimah High (Bellini and Massa, 1980). This uplift had some expression prior to the main late Silurian to early Devonian tectonics but underwent renewed growth during this time. An important consequence of the movements on this palaeohigh is that the Silurian section, including the hot shale source rock, is absent from the northeastern part of the Murzuq Basin, probably due to a combination of non-deposition and erosion following late Silurian to early Devonian reactivation (see Fig. 2 of this paper, Pallas 1980: Fig. 3, and Boote et al., 1998: Fig. 18b). The geological map of the Murzuq Basin in Fig. 1 shows the Silurian section to be absent from the southern margin of the Gargaf Uplift and also between latitudes 24 ~ to 25 ~ (approximately) on the eastern (Tibesti) flank of the basin.

Mid- to Late Carboniferous Tectonics Mid-Carboniferous compression resulted in reactivation of earlier high angle reverse/ transpressional faults, with the development of localised erosional unconformities that can be observed on seismic sections on the crests of uplifted fault blocks (e.g. Fig. 5 and 6) and by thickening of the Carboniferous sediments in the footwall sections. In the drilled hanging wall sections this unconformity approximates to the top of the Marar Formation. There is presently a lack of evidence from drilling results of any change in sedimentary facies where the Carboniferous section thickens across faults in the basin, there being no well penetrations on the footwall side of such faults. These mid Carboniferous fault movements resulted in significant modification and amplification of late Silurian to early Devonian compressional structures that later became hydrocarbon traps in the Murzuq Basin. Late Carboniferous 'Hercynian' compression, uplift and erosion played a significant role in the structural development of many of the North African petroleum producing basins (Boote et al., 1998), but available evidence indicates that this period of tectonism was less important in the development of the Murzuq Basin. No significant regional tilting occurred at this time in Murzuq, in marked contrast to the Illizi and Ghadames basins where a major angular

Chapter 14

307

unconformity is observed. However, despite the absence of demonstrable angular erosion, a significant stratigraphic section does appear to be missing at the 'Hercynian' unconformity in the Murzuq Basin, with several authors in agreement that all or most of the Permian section is not present (e.g. Mamgain, 1980; Abugares and Ramaekers, 1993; Boote et al., 1998). It is concluded herein that the basin was probably subject to significant regional uplift during late Carboniferous time, resulting in relatively uniform erosion of the Carboniferous succession and non-deposition of Permian sediments.

Mid-Cretaceous (Austrian) and Early Tertiary (Alpine) Tectonics In the centre of the Murzuq Basin there is a major unconformity between the Jurassic/early Cretaceous Mesak Formation sandstones and overlying Quaternary aeolian/fluvial deposits (Fig. 3). At the present time it is unclear whether this stratigraphic break is due to uplift during the early-mid Cretaceous 'Austrian' movements, the early-mid Tertiary 'Alpine' movements or a combination of both. Boote et al. (1998, Fig. 18b) suggest that both of these tectonic pulses caused uplift and erosion of the margins of the Murzuq Basin, but that the mid-Cretaceous pulse was responsible for the more significant movements. Compressional or transpressional fault reactivation is evident in NC 174, affecting the outcropping Mesak Formation (Figs 5 and 6), and dating this movement as post mid-Cretaceous. The geological map of Libya (IRC, 1985), as shown in simplified form in Fig. 1, shows that upper Cretaceous deposits in the northeast of the Murzuq Basin overstep earlier strata from the lower Cretaceous all the way down to the Cambrian on the Gargaf Uplift, demonstrating the effects of a major phase of uplift and erosion of approximately mid-Cretaceous age, probably associated with the Austrian tectonic phase. These vertical movements were regionally variable in nature, with the Gargaf Uplift being subject to significant uplift and exhumation while the basin centre was affected to a much lesser extent. This mid-Cretaceous event was demonstrably an important period of structural growth on the Gargaf Uplift and may also have played a significant role in the development of the Tihemboka High and the Tibesti High, which respectively form the western and eastern margins of the present-day basin. It remains enigmatic that the only identified tectonic movements in the Murzuq Basin during the Cretaceous were apparently compressional or transpressional in nature while the Sirte Basin, some 400 km to the northeast, was subjected to a major Cretaceous extensional rifting event. The upper Cretaceous sediments to the northeast of the Murzuq Basin comprise mixed carbonates and clastics of marine origin, which are dated as Maastrichtian in age (Woller, 1984). These sediments and the thin overlying Paleogene marine deposits now occur some 500 m above present-day sea level, clearly demonstrating the effects of renewed post-Paleogene uplift. It is inferred that this uplift is a consequence of the Alpine tectonic movements responsible for the growth of the Atlas Mountains in Morocco and Algeria. However, at the present time it is unclear whether this Tertiary uplift was uniform over the entire region or whether there was differential uplift of certain areas relative to others. Cretaceous and Tertiary tectonics were of prime importance in the development and modification of the hydrocarbon system in the Murzuq Basin. Differential uplift of the source rocks had a major effect on oil generation, with some areas possibly remaining within the oil window while oil generation in other areas ceased as the source rocks cooled. Existing oil traps were modified by fault reactivation, enhancing closure on some but promoting spillage or fault plane leakage on others. Migration pathways were also fundamentally altered by changes of regional dip direction. Unfortunately the timing and magnitude of the uplift events are not well documented at the present time, largely due to the absence of upper Cretaceous and Tertiary sediments over most of the basin. However the oil industry is currently attempting to address this

308

L. Davidson et al.

question with apatite fission track, fluid inclusion and other related studies which might eventually provide data to resolve questions on the relative timing of tectonics, oil generation and trapping formation, and also allow reconstruction of palaeomigration routes.

THE MAMUNIYAT FORMATION RESERVOIR Sandstone of the upper Ordovician Mamuniyat Formation forms the reservoir for the primary play in the Murzuq Basin, although in some wells the underlying Hawaz sandstone is also a productive reservoir where the Mamuniyat Formation is thin or absent. The Mamuniyat and the underlying Melaz Shuqran Formation were deposited at a time of glaciation over North Africa, which then lay along the margin of Gondwanaland. It is possible that at its maximum extent, the ice sheet extended across the whole of the Murzuq Basin. The depositional model for the Mamuniyat Formation is based on a combination of outcrop and well data. Field observations on the eastern and western margins of the basin (the Tibesti High and Tihemboka High respectively) indicate that the sediments appear to have been deposited in high-energy glacial outwash fans and channels, which spread out from the edge of the ice sheet (Beswetherick, 1992). The proximal parts of these fans include matrix-supported conglomerates containing boulder-sized clasts, whereas the distal parts are dominated by poorly sorted, coarse-grained sandstones deposited in braided fluvial conditions. Other sandstones were deposited as gravity driven flow deposits in a marine environment. All the outcrop evidence points to very rapid, high energy deposition and it is likely that the periods of deposition coincided with times of glacial retreat when large quantities of meltwater would be released to transport the glacially derived sediments towards the ocean to the north. On the northern margin of the basin (the Gargaf Uplift), the Mamuniyat Formation is dominated by moderate to well sorted, fine to medium grained, quartzitic sandstone, which tends to be both texturally and mineralogically mature. The bulk of the sandstone in this area appears to have been deposited in shallow marine conditions, although there is also some evidence for braided fluvial deposits, particularly near the base of the formation. Sedimentological analyses of cores taken from the early NC174 wells suggest that the Mamuniyat Formation was probably deposited in the broad setting of a sand rich, regionally extensive braid delta with alternating fluvial and marine influences controlled by glacially induced sea level changes. Sediment supply was from the south, with the shoreline trending roughly east to west. A range of facies can be identified. Thick sequences of clean sandstones occur in some wells and appear to have been deposited during marine destructive phases, involving reworking of the braid delta sediments in a high-energy marine environment. Sedimentation rates would have increased during times of glacial retreat when meltwater flow reached a maximum of volume and energy. This naturally coincided with periods of eustatic sea level rise, with the result that large volumes of sand were deposited as gravity flow deposits in a marine setting in front of the ice sheet. The upper part of the Ordovician section is occasionally seen in a facies association dominated by shale with thin sandstone beds. These thin sandstones were probably deposited in isolated channels and associated overbank environments on a relatively steep, unstable, mud-dominated slope that locally formed the delta front. Braided fluvial deposits appear to be relatively scarce in NC174. Correlation between drilled sections of the Mamuniyat Formation is severely hampered by two factors: 9 the lack of preserved biostratigraphic markers in the sand rich, high energy sediments, possibly masking diachroneity in the subcrop to the base Silurian unconformity, and 9 the highly heterogeneous, channelised nature of deposition, which resulted in wide variations in unit thickness (either due to original depositional controls or differential erosion) and rapid

Chapter 14

309

lateral changes of facies. Even in wells located only a few kilometres apart there can be a remarkable dissimilarity between well log patterns from the drilled sections of the Mamuniyat Formation. An obvious consequence of the heterogeneous nature of the Mamuniyat Formation is that reservoir simulation modelling is fraught with uncertainty. It may be the exception to find continuous reservoir units across any large field; more commonly the sand units are probably elongated in a channelised geometry, with older channels cut into by younger bodies, possibly in a complex anastamosing pattern (Smart, 2000). Channel orientation is an important factor and might best be determined through interpretation of 3D seismic data. However prediction of relative permeabilities in three dimensions, e.g. for design of a water flood, will require careful analysis of early production data. The expectation is that large scale permeability might be significantly different in the cross and along-channel orientations, but this fact will not be apparent from small-scale measurements taken from core samples. However the sand:shale ratio in the Mamuniyat Formation is generally high, averaging 0.8 (ignoring the reservoir potential of the sand), and where this ratio is high it is possible that good permeability might be present in all orientations.

ORDOVICIAN SANDSTONE PETROGRAPHY AND RESERVOIR QUALITY Petrographic studies indicate that the Ordovician sandstones cored in the NC174 wells are mainly fine to medium grained quartz arenites (S.RT., 1994). The detrital mineralogy is dominated by monocrystalline quartz grains, with minor amounts of polycrystalline quartz grains, lithic fragments, feldspar grains and mica. The petrographic studies allow the following diagenetic sequence to be proposed for the NC174 sandstones" 1. Precipitation of early grain coating clays, which are replaced by anatase, leucoxene and hematite, 2. The onset of quartz overgrowth development, 3. The main feldspar dissolution phase, and associated onset of kaolinite authigenesis, with continued quartz overgrowth, 4. The development of platy and fibrous illite, continued feldspar dissolution and kaolinite authigenesis, and 5. The precipitation of siderite and dolomite. Although the quartz overgrowths result in a reduction of porosity, they can also reduce the degree of compaction, preserving a well connected pore system and good permeability. Feldspar dissolution results in secondary porosity, which is in part offset by associated kaolinite authigenesis. Most of the sandstones contain at least small amounts of illite, which can block pore throats and reduce permeability. A few of the sandstones contain late-stage cements such as dolomite or siderite, which could be indicative of a marine depositional setting. There is no simple overall relationship between the porosity and permeability of the Ordovician sandstones in the NC 174 wells, although a relationship may exist in individual wells as illustrated in Fig. 7. The lack of a unique poroperm trend is probably a reflection of several factors: 9 Poor biostratigraphic control, leading to comparisons of sections of different ages, 9 Facies variations either within the fluvial system or between fluvial and marine units, 9 Wide variations in initial energy of deposition resulting in large differences in percentage of original clay content within the sands, 9 Different depths of maximum burial in different wells.

310

L. Davidson et al.

Figure 7. Porosity-permeability crossplot from analyses of Ordovician cored sections of exploration wells A1 to F1-NC174. Two distinct groupings are apparent, one with very good reservoir characteristics of moderate porosity and moderate to high permeability, the other with poor reservoir quality due to very low permeabilities. The primary differentiating factor seems to be percentage of original clay content, but other factors are also important. See text for discussion. The cored sections are interpreted as belonging to the Mamuniyat Formation, with the exception of C1-NC174 (uptight triangles), interpreted as belonging to the Hawaz Formation.

Chapter 14

311

The best reservoir quality sandstones in NC174 have measured porosities ranging from 12 to 20% combined with permeabilities of 100 to 1000 mD. The relatively low porosities, even in the clean sandstones, result from the abundant quartz overgrowths, but this has a less detrimental effect on permeability. Other wells in NC174 show significantly poorer reservoir quality, with low to moderate porosity (1 to 11%), and low permeability (less than 0.2 mD). The low porosity mainly results from the presence of late siderite, dolomite, and ferroan dolomite cements, whereas the low permeability results from the relative sparsity of macropores, and the presence of illite. Petrographic analyses indicate that the sandstones with the best reservoir quality are those in which the porosity mainly occurs in macropores, rather than micropores. These sandstones tend to contain relatively low amounts of kaolinite, and no or very little illite. The reservoir quality of the Mamuniyat sandstone depends on a combination of primary compositional and textural factors combined with secondary compactional and diagenetic factors. Reservoir quality tends to increase with increasing grain size and sorting, and decreasing detrital clay content (depositional fabric and the abundance of detrital feldspar, lithic fragments and mica are also important). These primary factors can be strongly overprinted by the secondary factors, which tend to reduce the porosity and permeability of the sandstone (for example, through compaction, precipitation of authigenic cement, and clay mineral authigenesis). One exception to this reduction in reservoir quality is that of feldspar dissolution, which can increase the porosity and permeability. Successful future prediction of the reservoir potential in the Mamuniyat Formation will only result from a detailed understanding of the environment of deposition and original facies distribution, together with better estimates of the subsequent degree of reworking and/or erosion that has occurred in the particular locations in question. These levels of predictive understanding have not been achieved to date. However it is anticipated that field development datasets of closely spaced wells and 3D seismic, which are now becoming available, should provide adequately constrained local models of reservoir quality distribution that might be extrapolated to the wider area of reservoir prediction in future exploration.

EARLY SILURIAN TANEZZUFT FORMATION 'HOT SHALE' SOURCE ROCK

Hot Shale Depositional Setting and Source Properties Oil-prone source rocks at the base of the Silurian section occur throughout much of North Africa. These source rocks have generated and expelled significant volumes of hydrocarbons, not only in the Murzuq Basin, but also in the nearby Illizi and Ghadames basins and in the Triassic basins of Algeria. Geochemical data in the Murzuq Basin indicate that the best source potential is provided by an irregularly distributed organic rich hot shale unit occurring within the lowermost part of the Silurian section. In the NC174 block, reliable biostratigraphic age determinations for the upper part of the Ordovician section are essentially limited to the early Ashgill dates obtained from the shale rich B 1-NC174 well (S.ET., 1994). The lower Tanezzuft shale in the NC174 wells is dated as early Silurian (early Llandovery). The major Silurian transgression initially resulted in a relatively shallow anoxic sea and the deposition of a thin transgressive sequence tract overlain by organic rich, lower Silurian hot shales with excellent source rock qualities. The anoxic conditions of deposition of the hot shales probably developed as a result of restricted marine circulation in shallow seas broken by numerous islands and peninsulas, the natural result of a low energy marine transgression over an irregular post-glacial topography. The early Silurian bottom waters were dense and very anoxic which, coupled with very low sedimentation rates, allowed the preservation of very high concentrations of organic matter.

312

L. Davidson et al.

Well logs through the lower part of the Tanezzufl section can be set alongside measured TOC percentage (Fig. 8), demonstrating the well known relationship between increasing TOC and gamma ray values (Rider, 1996). Resistivity also increases with rising TOC due to unexpelled hydrocarbons within the pore spaces. It is also apparent that as TOC increases there is a corresponding decrease in both sonic velocity and density, giving the useful relationship that increasing TOC in the Tanezzuft shale is associated with decreasing acoustic impedance. This relationship can be used for the prediction of the presence of hot shale through analysis of seismic reflection amplitude.

Figure 8. Logs from a typical NC174 well through the lower part of the Silurian Tanezzuft shale, including the hot shale interval. Measured TOC values are plotted alongside the logs to illustrate the relationship between log response and TOC. Note the strong positive correlations between TOC, GR and DT response, while TOC and density (RHOB) show a negative correlation. The high TOC hot shale source facies has significantly lower acoustic impedance than the low TOC shales.

Chapter 14

313

In most wells in the Murzuq Basin the transition downwards from normal to highly radioactive 'hot' Tanezzuft shale is gradual, as shown in Fig. 8, and therefore the top of the hot shale does not tend to produce a strong seismic reflector. However, the acoustic impedance of the hot shale is also significantly lower than the underlying Mamuniyat sandstones, and this sharp interface generates a large positive reflection coefficient, which produces a strong reflector on seismic data. In areas where the hot shale is not present and the normal shale rests directly on the Mamuniyat Formation sandstones, this reflector is significantly reduced in amplitude. This relationship allows the areal distribution of hot shale to be mapped using the seismic amplitudes of the base Silurian reflector calibrated to well control (Fig. 9). From the map of hot shale distribution (Fig. 9) it can be seen that this facies within the Tanezzuft Formation has a highly irregular distribution in NC174, probably as a result of the uneven topography of the transgressed surface. One implication of this observation is that no simple assumptions can be made regarding source rock distribution elsewhere in the basin. This uncertainty is further highlighted by Meister et al. (1991) who quote only one occurrence of the Silurian hot shale from a sample of twelve logged and geochemically analysed wells in the Murzuq Basin. Another implication is that the early Silurian transgression was diachronous on a local scale, with the oldest marine sediments (including the hot shale) being deposited only in topographic depressions, while the adjacent higher areas remained above sea level for some significant period of time, as illustrated by Ltining et al. (1999). The absence of hydrocarbon discoveries in large areas of the Murzuq Basin may be due to the source rock being absent or ineffective. The transition from deposition of hot to normal Silurian shale occurred as continued subsidence in the Murzuq Basin allowed deeper water conditions to become established and the circulation of open oceanic water destroyed the anoxic seabed conditions.

Figure 9. Distribution of the hot shale source rock within the Tanezzuft Formation in NC174. The map is based on well control, interpolated with mapping of variations in seismic amplitude. The base Tanezzuft reflector shows higher amplitude where the hot shale is present and lower amplitude where it is absent.

314

L. Davidson et al.

The hot shale unit is well developed in several of the NC174 wells, where it varies in thickness from 18 to 24 m, and gives gamma ray readings of up to 705 API units. Evidence of hydrocarbon generation is provided by high gas readings (C1 to C4) and crush cut fluorescence during drilling. A comprehensive series of geochemical analyses have been conducted on cuttings and sidewall cores from the Silurian and Cambro-Ordovician sections and on oil samples from NC174 wells. These analyses confirm the organic rich nature of the hot shale unit, with measured TOC values of up to 17% and pyrolysis yields ($2) of up to 64 kg/tonne. Visual kerogen analyses indicate that the organic matter is sapropelic and the samples plot on a Van Krevelen diagram as Type I/II (oil-prone). The remainder of the Silurian section, and the shales in the Cambro-Ordovician section tend to have little or no source potential and to be dominated by Type III, gas-prone kerogen. GC-MS analyses of an extract from core samples of the hot shale, and of the NC174 and NC115 oils indicate that the hot shale is a suitable source rock for having generated the oils (Geochem, 1993, 1994a, 1994b).

MATURITY MODELLING A programme of thermal modelling of the basal Tanezzufl hot shale source rock in the NC174 wells is currently being carried out by LGML using the BasinMod programme. Some early results of the models on the B 1-NC174 well are illustrated in Figs. 10, 11 and 12, but it is recognised that more controls are required to constrain the inputs to these models. In particular, the thermal history is poorly constrained, and this is currently being addressed by fluid inclusion and apatite fission track studies. The timing and amount of uplift during phases of compressional tectonics are also key inputs to the model, and these parameters are being refined by studies on basin tectonics combined with shale velocity work. A number of T Max calibration points derived from sample pyrolysis are available from within the Tanezzuft shale of the analysed well and these are used as calibration for the maturity model. The TMax data imply that the hot shale in the well reached mid-maturity (Ro = 0.9%) for oil generation at some point in geological time (Fig. 11). Work is currently being carried out with a view to improving the calibration of maturity modelling in the Murzuq Basin. Assumptions made for the illustrated model are:

Heat Flow The measured present-day heat flow, averaged for NC174 wells, of 45 mWm 2 was used throughout, with the exception of a short period of elevated heat flow (+ 20%) in the early Tertiary relating to the onset of volcanic activity in the Eocene in the Hoggar Massif, the Tibesti Massif and Jebel A1 Haruj, all of which surround the Murzuq Basin (Wilson and Guiraud, 1998).

Uplift and Erosion Approximately 300 m (1000 ft) of uplift and erosion has been assumed to have taken place both during the Carboniferous and Cretaceous/Tertiary compressive movements. Preliminary results from shale velocity studies support the assumption that maximum burial in the area of NC174 was at least 300 m deeper than present-day. It is recognised that the timing and magnitude of these uplifts are not well constrained, and work is currently in progress to provide more reliable quantitative estimates for these critical input parameters.

Chapter 14

315

It is apparent from Fig. 10 that the assumed amount of Carboniferous uplift and erosion would have had relatively little effect on source maturity, since the hot shale had only just entered the early mature window at the end Carboniferous. The results of the maturity model show that the hot shale in the B 1-NC174 well might have entered the mid-mature window and started to generate significant quantities of oil at about midCretaceous times, approximately 100 Ma B P, and continued to do so until early Tertiary, about 50 Ma BP, when uplift and erosion caused sufficient cooling of the source rock to remove it from the oil window (Fig 12). The hot shale in the modelled well is not generating any oil at present and will remain in this 'frozen' state unless its temperature is elevated by further burial or increased heat flow.

Figure 10. Burial history plot from preliminary maturity modelling of well B1-NC174 using the BasinMod programme. See text for assumptions and input data for this model.

316

L. Davidson et al.

It should be noted that the conclusions reached here do not agree with those of some other authors. Meister et al. (1991) took the view that the Murzuq Basin has not been subjected to significant post-Carboniferous erosion, that only the deepest parts of the present-day basin generated oil and that long distance migration is responsible for the oils trapped in shallower parts of the basin, e.g. in NC115. In contrast, Aziz (2000) proposes that the basin was subject to significantly more Permo/Carboniferous (Hercynian) and Tertiary (Alpine) uplift and erosion than suggested in this paper. The resulting maturity model for the Tanezzuft source rock

Figure 11. Calibration of the preliminary maturity model of the B 1-NC 174 well modelled in Fig. 10. The TMax values used as calibration points were derived from pyrolysis results from samples of Tanezzuft shale. No calibration points are available to constrain the model in the section younger than the Silurian.

Chapter 14

31'/

Figure 12. Calculated oil generation from well B 1-NC174 modelled in Fig. 10. The model indicates that significant oil generation began approximately 100 million years ago, but generation ceased at about 40 Ma due to uplift and cooling of the hot shale source rock. Note that the model does not distinguish between migrated oil and that retained within the source rock.

presented by Aziz (2000) indicates that oil generation began in the Carboniferous (preHercynian).

PRESERVATION OF TRAPPED OIL Although the Murzuq Basin oils show no obvious evidence of biodegradation, they all are characterised by a low to very low GOR and very low aromatic contents in the gasoline range fractions. This could be a primary effect caused by source rock composition, but is more likely to be a secondary effect produced by the removal of these lighter elements probably through the process of water washing. Tectonic uplift and erosion of the margins of the Murzuq Basin during the Cretaceous and/or Tertiary had the effect of exposing the basin aquifers to fresh water flushing, with the result that the Cambro-Ordovician sandstones are now charged with fresh water throughout the basin. Despite present arid conditions in the area, the Cambro-Ordovician aquifer is probably still full to its current structural spill point into the deeper younger basins to the north. Although this may be a relatively static condition at present, it is probable that significant hydrodynamic flow took place within this aquifer during the documented wet

318

L. Davidson et al.

climatic periods of 3000 to 12 000 years ago (Lutz and Lutz, 1995) and other undocumented wet periods before that, when the exposed aquifers to the east and west of the basin were continuously recharged with fresh meteoric water. Persistent exposure to this flowing fresh water could have resulted in dissolution of gas and the lighter liquid hydrocarbon elements from the trapped hydrocarbon accumulations within the Murzuq Basin.

THE ORDOVICIAN PLAY STATISTICS The Ordovician play has proved very successful in the Murzuq Basin. Approximately fifty-seven exploration wells have been drilled in the basin between 1958 and 1997, and nearly all of these were drilled with the primary objective of testing the Ordovician play. This exploration work has resulted in four large discoveries, with combined reserves of about 1000 to 1500 million barrels of recoverable oil, and sixteen smaller discoveries. The size of many of the discoveries is mainly a function of trap volume, although underfilling of traps may also be a factor. Of the remaining thirty-seven wells, twelve encountered hydrocarbon shows in Ordovician sandstones, and twenty-five were dry. One enigma of the Murzuq Basin involves several wells that have been drilled near the centre of the basin on valid structural closures at base Silurian level, but have failed to encounter any hydrocarbons. The D1-NC174 well (Fig. 6) is a good example. This well drilled a clearly defined fault-bounded structural closure, encountered a good hot shale source rock at the base of the Tanezzuft Formation and found excellent reservoir quality Mamuniyat sandstones beneath the base Silurian unconformity, but proved to be a dry hole with no shows in the reservoir section. At the time of writing the preferred explanation for failure of this well is that sandstone to sandstone juxtaposition across the main bounding fault resulted in cross-fault leakage, but a number of alternative explanations are also possible. Some other drilled structures in the basin have been found to contain only residual oil, perhaps implying leakage, and yet others appear to be underfilled. These examples serve to illustrate that the hydrocarbon system in the Murzuq Basin is not as simple as it may appear on initial analysis, and that significant work remains to be done to achieve a full understanding of the processes that controlled the initial accumulation and subsequent preservation of the major oilfields discovered to date. The drilling results for the Cambro-Ordovician play give an overall technical success rate of approximately 35% in the basin as a whole and 38% in NC174.

CONCLUSIONS The Murzuq Basin is a relatively underexplored basin in which reserves of approximately 1.5 billion barrels of recoverable oil have already been discovered. The primary play in the basin comprises an Ordovician periglacial sandstone reservoir, sourced with oil from and sealed by overlying Silurian shales. Oil was generated from an extremely rich source rock at the base of a thick Silurian shale section, and migrated directly into the underlying sandstone-rich Ordovician reservoir section. The timing of the main phase of oil generation is not well constrained but it is believed to have taken place from the mid-Cretaceous to early Tertiary. The relatively simple structure of the basin might have allowed long distance migration within the Ordovician sandstones, but late Cretaceous and early Tertiary tectonic movements have affected the basin subsequent to the main phase of oil generation, leading to obvious difficulties in prediction of original oil migration routes. Drilling results in the basin provide evidence that the play is not yet fully understood. In addition to the main discoveries there are also some drilled structures that appear never to have

Chapter 14

319

been charged with oil, while others contain only residual oil, perhaps implying leakage, and yet others are underfilled. These inconsistent results may be the result of the timing of oil generation predating at least one, and probably two phases of regional tectonic movements which may have caused spillage of oil from existing traps, re-migration from one trap to another and leakage of oil due to reactivation of trap-bounding faults. The uplift and erosion associated with these tectonic movements resulted in cooling of the source rocks which probably froze hydrocarbon generation in the basin from the early to mid-Tertiary. There is also uncertainty surrounding the areal distribution of the Silurian source rocks, which provided the bulk of the oil generated within the basin; the known distribution of the Tanezzuft hot shale is very irregular and it may be absent over large parts of the basin. The Ordovician play has met with only a limited amount of success elsewhere along the North African margin. In some basins, this lack of success can be attributed to the distribution of the source rock. However, in other cases, such as the neighbouring Illizi Basin of Algeria, the limited success of the play is mainly the result of a different tectonic history resulting in excessive burial of both the Ordovician reservoir and Silurian source. In contrast, the Murzuq Basin lies on a stable craton and has had a relatively gentle tectonic evolution, with the consequence that the Ordovician sandstone reservoir has not been buried to great depths and has retained good reservoir properties. Over large parts of the basin the Silurian source rock probably did not enter the oil window until Mesozoic times. The extent of late Cretaceous to early Tertiary uplift and associated erosion in the Murzuq Basin requires further quantification, but it is likely that the source rock in the centre of the basin is presently at least 300 m shallower than its maximum burial depth. However the properties of the trapped hydrocarbons indicate that pressure/temperature conditions of the mid to upper oil generation window were not exceeded. Two areas of study are currently being pursued in order to obtain a better understanding of the Ordovician play. Firstly, work is progressing to unravel the complex relationships between the generation/migration of oil and the various phases of tectonic movements that affected the area. Secondly, a better model of the Ordovician reservoir distribution and quality is being developed by analysis of recent wells drilled in the basin, coupled with seismic stratigraphic interpretation. The conclusions from these studies should greatly assist future exploration in the basin.

ACKNOWLEDGMENTS The authors thank our numerous colleagues and co-workers whose efforts over the years have contributed to the results and interpretations expressed in this chapter. We also thank those who have discussed and constructively reviewed earlier versions of this text. This paper is published with the kind permission of the National Oil Corporation and our Joint Venture partners in NC174: Agip North Africa BV and the Korea National Oil Corporation.

REFERENCES ABUGARES, Y.I. and RAMAEKERS, E (1993). Short notes and guidebook on the Palaeozoic geology of the Ghat area, SW Libya; Field trip, October 14-17, 1993. Earth Sci. Soc. Libya, Interprint Ltd., Malta, 84 p. AZIZ, A. (2000). Stratigraphy and hydrocarbon potential of the Lower Palaeozoic succession of License NC-115, Murzuq Basin, SW Libya. This volume. BELLINI, E. and MASSA, D. (1980). A stratigraphic contribution to the Palaeozoic of the southern basins of Libya. In: The Geology of Libya, M.J. Salem and M.T. Busrewil (Eds). Academic Press, London, I, 3-56. BESWETHERICK, S. (1992). Report on a Field Trip to the southeastern and northern (Gargaf Arch) margins of the Murzuq Basin. LGML Internal Report.

320

L. Davidson et al.

BESWETHERICK, S., HIMMALI, A. and JHO, J.S. (1996). Report on a Field Trip to the Tihemboka High and Gargaf Arch, Murzuq Basin, Southwest Libya. LGML Internal Report. BOOTE, D.R.D., CLARK-LOWES, D.D. and TRAUT, M.W. (1998). Palaeozoic petroleum systems in North Africa, In: Petroleum Geology of North Africa, D.S. Macgregor, R.T.J. Moody and D.D. ClarkLowes (Eds). Geol. Soc. Lond. Spec. Publ., 132, 7-68. ECHIKH, K. (1998). Geology and hydrocarbon occurrences in the Ghadames Basin, Algeria, Tunisia, Libya. In: Petroleum Geology of North Africa, D.S. Macgregor, R.T.J. Moody and D.D. Clark-Lowes (Eds) Geol. Soc. Lond. Spec. Publ., 132, 109-129. GEOCHEM (1993). Oil to oil correlation, Murzuq Basin, Libya. Report prepared for LGML. GEOCHEM (1994a). Geochemical Evaluation of the DST-1 crude oil from Well A1-NC174, North Scorpion Field, Murzuq Basin, Libya. Report prepared for LGML. GEOCHEM (1994b). A geochemical study involving samples from the NC174 Block in the Murzuq Basin of southwest Libya. Report prepared for LGML. GLOVER, R.T. (1999). Aspects of intraplate deformation in the Saharan cratonic Basins. Ph.D. Thesis. University of Wales, Aberystwyth, 206 p. HADLEY, D.E (1992). Sedimentology and facies of the Mamuniyat Formation of southwestern Libya. LGML Internal Report. I.R.C. (1985). Geological Map of Socialist People's Libyan Arab Jamahiriya. Scale: 1:1 000 000. Geol. Res. Mining Dept., Tripoli. KLITZSCH, E. (1971). The structural development of parts of Africa since Cambrian time. In: Symposium on the geology of Libya, C. Gray (Ed.). Fac. Sci. Univ. Libya, Tripoli, 253-262. LUNING, S., CRAIG, J., FITCHES, W.R., MAYOUF, J., BUSREWIL, A., EL DIEB, M., GAMMUDI, A., LOYDELL, D. and MCILROY, D. (1999). Re-evaluation of the petroleum potential of the Kufra Basin (SE Libya, NE Chad): does the source rock barrier fall? Mar. Petrol. Geol., 16, 693-718. LUTZ, R. and LUTZ, G. (1995). The Secret of the Desert. Golf Verlag, Innsbruck, 177 p. MAMGAIN, V.D. (1980). The Pre-Mesozoic (Precambrian to Palaeozoic) stratigraphy of Libya, a reappraisal. Dept. Geol. Res. Min. Bull., Tripoli, 14, 104 p. MEISTER, E.M., ORTIZ, E.E, PIEROBON, E.S.T., ARRUDA, A.A. and OLIVEIRA, M.A.M. (1991). The origin and migration fairways of petroleum in the Murzuq Basin, Libya: an alternative exploration model. In: The Geology of Libya, M.J. Salem, M.T. Busrewil and A.M. Ben Ashour (Eds). Elsevier, Amsterdam, VII, 2725-2741. PALLAS, E (1980). Water resources of the Socialist People's Libyan Arab Jamahiriya. In: The Geology of Libya, M.J. Salem and M.J. Busrewil (Eds). Academic Press, London, II, 539-574 p. PIEROBON, E.S.T. (1991). Contribution to the stratigraphy of the Murzuq Basin, SW Libya. In: The Geology of Libya, M.J. Salem and M.N. Belaid (Eds). Elsevier, Amsterdam, V, 1767-1783. RIDER, M. (1996). The geological interpretation of well logs. 2nd Edition. Whittles Publishing, London, 280 p. SMART, J.D.C. (2000). Seismic expressions of depositional processes in the upper Ordovician succession of the Murzuq Basin, SW Libya. This volume. S.ET. (1994). Sedimentology of the Ordovician Sandstones in Block NC174, Murzuq Basin, Libya. Simon Petroleum Technology (Robertson Research International) Report prepared for LGML. VOS, R.G. (1981). Sedimentology of an Ordovician fan complex, western Libya. Sediment. Geol., 29, 153-170. WILSON, W. and GUIRAUD, R. (1998). Late Permian to recent magmatic activity on the AfricanArabian margin of Tethys. In: Petroleum Geology of North Africa, D.S. Macgregor, R.T.J Moody and D.D. Clark-Lowes (Eds). Geol. Soc. Lond. Spec. Publ., 132, 231-263. WOLLER, E (1984). Geological map of Libya, 1:250 000. Sheet: A1 Fuqaha (NG 333). Explanatory Booklet. Ind. Res. Cent., Tripoli, 123 p.